The identification and characterization of cell surface molecules in the hematopoietic system provides insight into patterns of cellular communication and the role of these molecules in the regulation of the immune response. This project will focus on the study of HB15, a cell surface glycoprotein identified by cDNA cloning and monoclonal antibody (mAb) production and characterization. HB15 demonstrates a unique pattern of expression, being found primarily in lymphoid tissue, and restricted to cells critical in antigen (Ag)-presentation. Strong expression of HB15 has been identified on dendritic cells, Langerhans cells and Reed- Sternberg cells. Dendritic cells are potent Ag-presenting cells that are believed to be important in the activation of both cytotoxic and helper T cells. Structurally, HB15 has a single immunoglobulin (Ig)-like domain, suggesting that HB15 may be important in cellular communication. The goals of this study will be as follows: (i) To characterize the structure and function of the human and murine HB 15 gene products. The initial work will involve the molecular cloning of the murine cDNA for HB 15, and determination of the human and mouse gene structure. Identification of homologous regions should predict functional importance of the molecule and provide insight into IG gene conservation through evolution. (2) To characterize the expression pattern of this molecule in hematopoietic and histiocytic malignancies, and in particular, in Hodgkins Disease. Improved methods of dendritic cell isolation and purification will be developed using the HB 15 mAb and should provide a means for classification and characterization of the family of dendritic cells. Indirect immunofluorescence and flow cytometry analysis utilizing HB 15 mAb will be used to investigate the contribution of dendritic cells to lymphocyte activation in autoimmune disorders. (3) To characterize the function of the cells that express HB15, the role of those cells in immune dysfunction, and how HB15 is involved in the function of those cells. Dendritic cell function will be measured through assays of accessory cell function in the mixed leukocyte reaction, and the ability of HB15 mAb to abrogate this function will be tested. The ability of HB15 to provide a co-stimulatory signal for T cell proliferation will be investigated in in vitro assays. Construction of HB15-Ig fusion proteins and adhesion assays will be utilized to identify the natural ligand for HB15. These studies will provide a comprehensive effort to understand the, role of HB 15 in normal and abnormal lymphocyte biology. They will contribute to the phenotypic and functional characterization of dendritic cells and their accessory cell family members. An understanding of this molecule and the precise function it serves in the biology of lymphocyte activation will provide the basis for developing rational tools to modulate the mechanisms of lymphocyte activation in states of immune dysregulation.